The present invention relates to an apparatus for measuring the characteristics of electronic devices, such as transistors, diodes, etc.
An electronic device measurement apparatus, generally known as a curve tracer, is useful to measure characteristics of electronic devices such as transistors, diodes, etc. A conventional electronic device measurement apparatus of this type is illustrated in FIG. 1, wherein an AC voltage from a line source is applied through a switch 10 to power supply circuit 12 as well as to variable transformer 14. The power supply circuit 12 produces DC voltages for each circuit of the measurement apparatus, while the variable transformer 14 supplies an AC voltage having a controlled amplitude to a primary winding of transformer 16. A plurality of taps are provided on the secondary winding of the transformer 16, and one of these taps, excluding the lowest position tap, is selected by a switch 18 for connection to the anode of diode 20 which half-wave rectifies the selected AC voltage. The elements 14-20 provide a collector supply circuit 21. The cathode voltage of diode 20 is applied to a first terminal of a device under test (DUT), e.g., to the collector of a transistor 26 through a load resistor 24 selected by a switch 22. The base of the transistor 26 receives a stepped bias signal from a bias supply circuit 28, while the emitter thereof, comprising a second terminal of the DUT, is grounded. The lowest tap of transformer 16 is connected to the emitter of the transistor 26 through a collector current detector resistor 32 as selected by a switch 30. Furthermore, a voltage divider 36 is selected by switch 34 and divides down the collector voltage Vc of the transistor 26 for application to a horizontal deflection plate of cathode ray tube (CRT) 40 via an amplifier 38 having a high input impedance. Differential amplifier 42 operates as a high input impedance voltage detector which detects a voltage across a resistor 32 (being substantially proportional to the collector current Ic) and couples this voltage to a vertical deflection plate of CRT 40. The CRT 40 displays the Vc-Ic characteristic curve of the transistor 26 as shown in FIG. 2, as the voltage thereacross is varied in response to half cycles of line voltage. In FIG. 1, the DUT 26 comprises a common emitter connected transistor. However, another electrode of the transistor may be grounded, or the DUT may comprise another electronic device such as a diode. In any case, the voltage-current characteristic of the DUT is displayed on CRT 40.
The conventional measurement apparatus as shown in FIG. 1 has many disadvantages. For example, the collector supply circuit consisting of elements 14-20 uses the external line voltage directly, the line voltage waveform desirably comprising a sine-wave. It should be noted, however, that a pure sine-wave consists of only one frequency component while the waveform of the external line voltage is not a pure sine-wave, i.e., it is not a symmetrically repeated waveform, but rather includes various distortions. In other words, the repeated voltage waveform applied to the DUT, not being a pure sine-wave, is not symmetrical but rather contains differing frequency components, and it is found a forward trace of the characteristic curve displayed on the CRT 40 (during a rising period of the rectified sine-wave voltage) is different from a back trace thereof (during a falling period of the rectified sine-wave voltage). Therefore, the characteristic of the DUT cannot be measured exactly, but a phenomenon occurs called display distortion in this specification. Moreover, the peak amplitude of the line voltage is not constant but varies within a predetermined range. As a result of the variation, the peak amplitude of the repeat waveform voltage applied to the DUT varies in response to the line voltage, producing further inaccuracies in measurement.
If a digital storage circuit is added to the measurement apparatus shown in FIG. 1, by inserting additional circuits between the amplifier 38 and the CRT 40 and between the amplifier 42 and the CRT 40, with each of the additional circuits consisting of an analog-to-digital converter, a digital memory and a digital-to-analog converter, the output from the analog-to-digital converters may be affected by ripple of the power supply circuit 12 since the clock frequency for the analog-to-digital converters is independent of line voltage frequency. Therefore measurement accuracy is further reduced. For synchronizing the clock frequency of an analog-to-digital converter with the line frequency, an additional phase control circuit would be needed, resulting in additional expense in the measurement apparatus.
It should also be noted that switches 30 and 34 are controlled in accordance with the desired measurement range. Thus, horizontal-axis "size" of the display is determined by the dividing ratio of the voltage divider 36 selected by switch 34 and the vertical-axis "size" is determined by the value of the resistor 32 selected by switch 30. The maximum voltage to be applied to the DUT, or the measurement range, is determined by the adjustment of the variable transformer 14 and the selection of switch 18. It will be seen it is necessary to select a resistor 32 having a small value when measuring a large current flowing through the DUT, and it is necessary to select a voltage divider 36 having a large dividing ratio when measuring a large voltage across the DUT.
When the switches 30 and 34 select a resistor 32 of small value and a voltage divider 36 of large dividing ratio respectively, the CRT 40 may, for example, be able to display a characteristic curve within a window defined by dotted line 46 shown in FIG. 2. If the output amplitude of the variable transformer 14 is adjusted to be relatively large, the characteristic curve may be displayed on all of the screen area of the CRT 40. In this instance, the selected resistor 32 and the resistors of the selected voltage divider 36 should be resistors which can withstand a large current and a high voltage. When the switches 30 and 34 select respectively a resistor 32 of higher value and a voltage divider 36 of smaller dividing ratio, a display window of the CRT 40 is selected which may be limited within a window defined by a dotted line 50 shown in FIG. 2. Thus, for a given current through a larger resistor 32, larger voltages are generated thereacross and less of the characteristic may be viewed on the CRT. When the operator wants to measure the DUT only within the window 50, the resistor 32 and the resistors of the voltage divider 36 can be resistors appropriate to withstand a relatively small current and a relatively small voltage. Thus, the voltage and current applied to the DUT from transformer 14 need not be as high. However, it is still possible to apply high voltage and current to the DUT within a range of values larger than window 50, e.g., within window 46, by adjusting variable transformer 14. Therefore, the conventional measurement apparatus must utilize resistors which can withstand a high voltage and a high current for all the resistors 32 as well as for all resistors of the voltage dividers 36. If relays are used for switches 30 and 34, all the relays must be able to withstand high voltages and large currents. Resistors and relays satisfying these requirements are large and expensive in construction. Moreover, the DUT may receive unnecessarily high voltages and currents. It should also be noted that even though the variable transformer 14 is further adjusted to increase the voltage and current to the DUT to points in the window 46 as noted above, the CRT 40 will ordinarily display the characteristic curve only within window 50 and not outside thereof because the values of the resistor 32 and the voltage divider 36 are suitably employed to determine display sensitivity, and the display area of the CRT 40 is limited.
As described hereinbefore, the voltage divider 36 is utilized in adjusting the measurement range. As a result, the current flowing through the selected resistor 32 is the sum of the collector current of transistor 26 and the current flowing through voltage divider 36. It should be further noted that the base current of the transistor 26 flows through ground to the bias supply circuit 28 and does not flow through resistor 32, because the output current value of the collector supply circuit 21 (at the cathode of diode 20) is equal to the input current value thereof (at the lowest position terminal of transformer 16). In any case, the voltage across the resistor 32 is not directly proportional to the collector current of transistor 26 and this leads to error in the measurement result.
The above-described disadvantages of the conventional measurement apparatus are summarized as follows:
1. Since the line voltage is applied to the DUT through a transformer, the displayed characteristic curve is distorted, and the ripple of the power supply circuit affects the accuracy of an analog-to-digital converter if one is used.
2. The collector current detecting resistors, the voltage dividers and the switches must withstand high voltage and large currents regardless of the measurement range.
3. The current flowing through the DUT is not detected accurately because of a voltage divider shunted thereacross.